BACKGROUND: Cognitive-behavioural therapy (CBT) is effective for treating anxiety and depression in primary care, but there is a shortage of therapists. Computer-delivered treatment may be a viable alternative. AIMS: To assess the cost-effectiveness of computer-delivered CBT. METHOD: A sample of people with depression or anxiety were randomised to usual care (n=128) or computer-delivered CBT (n=146). Costs were available for 123 and 138 participants, respectively. Costs and depression scores were combined using the net benefit approach. RESULTS: Service costs were 40 British pounds (90% CI - 28 British pounds to 148 British pounds) higher over 8 months for computer-delivered CBT. Lost-employment costs were 407 British pounds (90% CI 196 British pounds to 586 British pounds) less for this group. Valuing a 1-unit improvement on the Beck Depression Inventory at 40 British pounds, there is an 81% chance that computer-delivered CBT is cost-effective, and it revealed a highly competitive cost per quality-adjusted life year. CONCLUSIONS: Computer-delivered CBT has a high probability of being cost-effective, even if a modest value is placed on unit improvements in depression.